Conventionally, a lithium-ion battery is housed in a metal case, e.g. a steel can, or an aluminum can, and a polymer battery is packaged in an aluminum laminated film. These batteries need to be further externally packaged to protect them against shock from dropping or other external impact. There are known methods for externally packaging the battery body, for example, a method wherein the battery body is hermetically covered with a case-shaped member formed of an aluminum sheet, a plastic material, etc., and a method wherein the outer surface of the battery body is covered with a thermoplastic resin by injection molding, thereby packaging the battery body in the molded resin.
It is also publicly known that heat-shrinkable tubing made of polyvinyl chloride or the like is used as a heat-shrinkable synthetic resin material for covering batteries. Stretched tubing made of a mixed composition consisting essentially of an olefin-based ionomer resin is also publicly known as an example of heat-shrinkable tubing [for example, see Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-170365].
However, the conventional externally packaging methods have not yet satisfied all the demands but still have many problems to be solved. Particularly, cellular phones and the like require as thin a battery as possible, and a battery that meets the requirement has appeared. If this battery is covered with thin hear-shrinkable tubing, the tubing may be partially wrinkled. Therefore, there is a problem in terms of reliability in mass production of batteries of stable quality. Meanwhile, injection molding is excellent in mass productivity but disadvantageous for the following reason. With the conventional injection molding, the flow of molten resin is deteriorated as the thickness of the molded part to be produced is reduced. Accordingly, there is a limit to the reduction of the part thickness.
Cellular phones and the like require a material having chemical resistance and fire retardance. To prepare a resin material satisfying these requirements, PBT (polybutylene terephthalate) resin may be mixed with a fire retardant consisting essentially of an inorganic material, for example. In this case, a part thickness of at least 0.3 mm to 0.4 mm is required because the flow of molten resin would otherwise be deteriorated. Thus, it is even more difficult to obtain a thin molded part. Accordingly, the thickness of injection-molded parts cannot be reduced to less than a predetermined thickness as long as an injection molding method based on the presently common technique is employed. Lithium-ion polymer secondary batteries, in particular, are characterized by their capability of implementing a thin and lightweight structure. Therefore, a thick outer casing that would lessen the advantageous feature of lithium-ion polymer secondary batteries cannot be adopted.
The outer casing for lithium-ion polymer secondary batteries is required to exhibit stable mechanical strength despite the thin and lightweight structure. Meanwhile, there has been proposed a method of producing a thin-walled molded part for housing an electronic component, wherein a thin-walled portion of the molded part is formed of a film, and a thick-walled portion thereof is formed by injection molding [Japanese Patent Application Unexamined Publication (KOKAI) No. 2002-283507]. However, the thin-walled molded part does not completely cover the outer surface of the housed component. Therefore, it cannot be employed for the above-described outer packaging of batteries.